China broke with its close ally North Korea yesterday and said the world must hit the country with “punitive actions” because of its nuclear-bomb test.

China’s ambassador to the United Nations Security Council said the world body should deliver a “firm, constructive, appropriate but prudent response to North Korea’s nuclear threat.”

Wang Guangya stopped short of saying which sanctions pushed by the United States would get crucial support from Beijing.

“I think that there has to be some punitive actions but also I think that these actions have to be appropriate,” Wang told reporters.

China is North Korea’s closest ally and biggest trading partner – and had been the strongest supporter of trying to head off a North Korean nuclear arsenal through negotiations.

Wang’s tough comments came amid other developments:

* U.S. Ambassador John Bolton said the Security Council hadn’t nailed down an agreement on sanctions against Pyongyang yet, “but we’re making progress.”

* North Korea made a veiled threat to fire a nuclear-tipped missile if the crisis with the United States escalates.

* A North Korea diplomat admitted that the nuclear test was “smaller . . . than expected.”

“Small-scale success means that large-scale success is possible,” he told the South Korean newspaper Hakyoreh, which has good ties with North Korea.

* The White House suggested the test was more boast than blast and North Korean had detonated “very old” arms, not nuclear weapons.

That was buttressed by suggestions by scientists that the 4.2 Richter scale readings reported after the Sunday test could have been produced by conventional weapons, not nukes.

Intelligence analysts said the most likely scenario is that North Korea did what they claimed – creating a chain reaction to detonate a plutonium-based bomb.

But the test still seemed to be a bit of a flop.

“The working assumption in the intelligence community is that it was a nuclear test that didn’t go well,” said a U.S. official.

Initial reading indicated the blast impact was the equivalent of 1,000 to 15,000 tons of TNT.

But yesterday U.S. officials said it might be a fraction of that.

“I’d go in the direction of 200 tons. That’s what’s suggested by the seismic readings,” one official said.

Experts said it was highly unusual for a country’s first nuclear test to register such small readings. That could be explained by the plutonium only partially detonating or by a misjudgment in timing the blast, they said.

Also, the impact of the blast, at Gilju in Hamgyong Province, might have been heavily muffled by underground surroundings.

The North Korean diplomat quoted by Hakyoreh said his country could take “additional measures” to follow up the test.

That reinforced the theory that the communist state was trying to prove it had joined the nuclear club even if it meant just getting inside the front door.

“I think they got a partial result,” said Philip Coyle, a senior adviser to the Center for Defense Information, a think tank in Washington.

“For them it was enough . . . to say that it was a success. It helps them to claim that they are a nuclear power, and that the world should take them seriously, which is what they want.”

“But I wouldn’t be surprised if after several months they don’t try again,” Coyle said.

Anton Khlopkov of Moscow’s Center for Policy Studies said North Korea may have intended a small blast to conserve on its stash of plutonium.

He said even an imperfect test showed how far North Korea had advanced.

“This is a success for them, even if they have some problems with the power of a nuclear test,” Khlopkov said.

“If even such a poor country with a failing economy like North Korea can make a nuclear device, then we should redouble our efforts to strengthen the nonproliferation regime.”

But the White House raised doubts about the kooky Kim’s boast of becoming a nuclear-armed power.

Presidential spokesman Tony Snow noted that North Korea claim would mean it had made huge progress only two years after booting international weapons inspectors from their country.

“You seriously believe that they have actually done everything within two years?” he told reporters.

“You could have something that is very old and off the shelf here, as well, in which case they’ve dusted off something that is old and dormant,” Snow added.

He said confirmation is difficult and there’s “a remote possibility that we’ll never know.”

“We need to find out precisely what it is that took place yesterday, and that is something that’s going to take a while for the scientists and other to work through,” Snow said.

The situation was different in July when the North tested its long-range missile and it was quickly determined that it fizzled shortly after takeoff.

Meanwhile, Pakistan denied that the disgraced former head of its nuclear weapons program was the farther of the North Korean bomb.

Abdul Qadeer Khan is widely suspected of providing Pyongyang with uranium enrichment centrifuges along with his own considerable expertise in the 1990s.

But a foreign ministry spokeswoman said yesterday, “There is absolutely no link between what happened yesterday and what A.Q. Khan might have had to do with North Korea. Our nuclear program is uranium-based and North Korea’s is plutonium-based.”

The North also made a chilling nuclear threat yesterday.

“We hope the situation will be resolved before an unfortunate incident of us firing a nuclear missile comes,” an unidentified North Korean official told the Yonhap news agency. “That depends on how the U.S. will act.”

The official said the nuclear test was “an expression of our intention to face the United States across the negotiating table.”

A North Korean official, Ri Johng-hyok, said the bomb was a necessary “nuclear deterrent against the Americans.”

It was a “matter of life and death,” he said on a visit to Belgium.

A South Korean seismologist said the North would likely conduct a second test to verify the valuable information gleaned from the first test.

“They will not end the test after one experience,” Chi Heon-cheol said.

The U.S. military in the Far East kept a low profile yesterday as Washington stressed the need for a diplomatic solution.